Rotary steerable drilling systems are used in many types of drilling applications to control the direction of drilling. Directional control has become increasingly more prevalent during drilling of subterranean oil and gas wells, for example, to more fully exploit hydrocarbon reservoirs. In some cases, rotary steerable drilling systems are used to drill wells with horizontal and deviated profiles.
To drill directional boreholes into subterranean formations, operators generally employ a bottom hole assembly (BHA) connected to the end of a tubular drill string, which is rotatably driven by a drilling rig from the surface. The drilling rig provides the motive force for rotating the drill string and also supplies a drilling fluid under pressure through the tubular drill string to the BHA. To achieve directional control during drilling, the BHA may include one or more drill collars, one or more stabilizers and a rotary steerable drilling system positioned above the drill bit, which is the lowermost component of the BHA. The rotary steerable drilling system generally includes a steering section and an electronics section and other devices to control the rotary steerable drilling system.
Rotary steerable drilling systems are often classified as either “point-the-bit” or “push-the-bit” systems. In point-the-bit systems, the rotational axis of the drill bit is deviated from the longitudinal axis of the drill string generally in the direction of the new hole. The new hole is propagated in accordance with a three-point geometry defined by upper and lower stabilizer touch points and the drill bit. The angle of deviation of the drill bit axis, coupled with a finite distance between the drill bit and the lower stabilizer, results in a non-collinear condition that generates a curved hole. There are many ways in which this non-collinear condition may be achieved, including a fixed bend at a point in the BHA close to the lower stabilizer or a flexure of the drill bit drive shaft distributed between the upper and lower stabilizer.
In push-the-bit systems, typically no mechanism deviates the drill bit axis from the longitudinal axis of the drill string. Instead, the non-collinear condition is achieved by causing either or both of the upper and lower stabilizers, for example via pads or pistons, to apply an eccentric force or displacement to the BHA to move the drill bit in the desired path. Steering is achieved by creating a non-collinear condition between the drill bit and at least two other touch points, such as the upper and lower stabilizers, for example.
Despite such distinctions between point-the-bit and push-the-bit systems, an analysis of their hole propagation properties reveals that facets of both types of systems are present during operation of each type of rotary steerable drilling system. More recently, hybrid rotary steerable drilling systems have been introduced that intentionally combine the structure and functionality of both the classical point-the-bit system and the classical push-the-bit system into a single system by design rather than circumstance.